Selective transmission network



March 6, 1934. P. Q FARNHAM 1,950,358

SELECTIVE TRANSMISSION NETWORK Filed April 4, 1933 (ff-glia Patented Mar. 6, i934 SELECTIVE TRANSMSSION NETWORK Paul 0. Farnham, Boonton, N. J., assigner to Radio Frequency Laboratories, Incorporated, Boonton, N. J. a corporation of New Jersey Application April 4, 1933, Serial No. 654,431

23 Claims.

This invention relates tc selective transmission networks and more particularly to selective networks suitable for use in radio receivers of the superheterodyne type.

While not limited thereto, the invention will be described with reference to a superheterodyne receiver, in which it operates as an image supressor. It is well known that carrier waves of two frequencies will beat with local oscillations in a superheterodyne to produce the same intermediate frequency when two carrier frequencies are spaced at equal frequency intervals from, and one on either side of, the local oscillator frequency. |Thus when, as is the usual case, it is l desired to receive only one of these two carrier waves, the receiving system prior to the local oscillator and first detector istuned to respond to this desired frequency and sufficient selectivity against the other or undesired carrier wave must be provided to prevent interference therefrom. This undesired carrier frequency is commonly termed the image frequency.

Selectivity effective to suppress the image frequency may be obtained by cascading tuned radio frequency stages in advance of the first detector or by a coupled circuit network comprising single circuits each tuned to substantially the same frequency. Since adequate selectivity against carrier waves of other than the image frequency is provided in the intermediate frequency amplier, it not economical t include a large number of conventional tuned radio circuits ahead of the first detector of a superheterodyne receiver.

In accordance with the present invention, the coupling system between the antenna or collector structure and the rst vacuum tube of a superheterodyne receiver, or between two vacnum tubes, is so designed as to obtain with a single tuned circuit an image suppression factor equivalent to that given by two conventional tuned radio circuits.

The invention is equally useful in any alternating current transmission system, either tuned or untuned, which is to eect a maximum transmission at one frequency and to provide maximum attenuation to Waves of a different frequency.

An object of the invention is to provide an improved and more efcient coupling system of the type which effects a maximum transmission at one frequency and a maximum suppression at another frequency. An object is to provide a network including a simple coupling link additional to the conventional tuned coupling circuit of a radio frequency transmission system,

(Cl. FTS-A4) the additional coupling link having the property of effecting a maximum attenuation at a frequ '.icy differing in a predetermined amount from the frequency for which the system gives maximum transmission. A further specific ob- V53 ject is to provide a superheterodyne receiver system in which the particular image frequency corresponding to a desired frequency will be substantially attenuated when the coupling system tuned to the desired frequency. Still another object is to provide a radio frequency coupling system tunable over a band of frequencies by means of a single tunable element and having maximum attenuation at a frequency which is spaced from the frequency of maximum transl@ mission by a substantially constant frequency difference.

These and other objects and advantages of the invention will be apparent from the following specification, when taken with the accom- *Y panying drawing, in which Figs. 1 and la are circuit diagrams of coupling systems contemplated by the invention;

Fig. 2 is a circuit diagram illustrating an application of the system shown in Fig. 1 to an 80 antenna input circuit;

Fig. S is a circuit diagram illustrating the application of the circuit shown in Fig. l to a typical tuned radio amplifying stage feeding the first detector of a superheterodyne receiver; and

Fig. 4 is a central section through a coil assembly adapted for the circuit shown schematically in Fig. 3.

In the several views of the drawing, the reference characters A, A and B, B identify the pairs of terminals at opposite sides of the coupling system and, as illustrated, the terminals A and B of the sets are at the same carrier Wave potential which, in the usual case, is ground or zero potential. For convenience of explanation, the terminals A, A will be designated as the input terminals of the system, and B, B as the output terminals, but it is to be noted that the coupling system is a passive network and that the reciprocal relationships existing in such networks make it possible to reverse the direction of transmission without affecting the selective properties of the coupling system.

Like reference characters are applied to similar circuit elements in the several views and, referring particularly to Fig. l, a coil L1 is connected between the input terminals A, A and coupled magnetically to both of the coils L2, L3 which are serially connected between the output terminals B, B; the coil L2 being shunted by 110 a condenser C2 which may be, and usually is, variable for tuning the circuit L2, C2 to resonate at the desired carrier frequency. This resonant Circuit also includes resistance R2 which may consist solely of the inherent resistance of the tuned circuit, or may include an added physical resistor. The magnetic couplings between coil L1 and the coils L2, La are indicated graphically as M2, M3, respectively, and capacitive couplingis provided by a small condenser C1 which is connected between the high-potential terminal A of coil L1 and the junction of the coils L2, L3. As shown in the more detailed circuit diagrams, the common, low-potential terminals A', B are customarily grounded when the coupling system forms part of a radio transmission system but the ground connection may be unecessary in some systems and therefore is not shown in Fig. 1.

The total output voltage E developed between the output terminals B, B is made up of the sum of the series voltages E2, E3 which are developed across L2, C2 and L2, respectively. An analysis of the circuit operation will indicate .how the proper choice of circuit constants results in a network that functions to suppress the transmission of a frequency which differs, in a predetermined relationship, from the desired transmission frequency.

If a driving voltage at input terminals A, A1 is assumed to produce a current in coil L1 which is indicated by the arrow i1, the equations may be written for voltages E2, E3 when C2 is adjusted for a maximum voltage E2 at the given impressed frequency. If the impressed frequency of i1 is varied from this value, leaving C2 fixed, it will be found that the expressions for E2 and E3 become of opposite sign for frequencies on one side of the initial value, while on the other side of that initial value they have the same sign. At the initial frequency at which C2 is tuned for maximum E2, the voltage E2 is in quadrature and small compared to E2. It is apparent that the system will give maximum attenuation at a frequency for which E3 and E2 have opposite signs and equal magnitude.

Let wzimpressed angular frequency w1=impressed angular frequency for maximum E2.

At any value of impressed frequency, neglecting the effect of C1,

Let 22=22L2C2 and 112=R22C2; this relationship becomes:

1 2 E2 JwMm 122+J'fl2 When condenser C2 is adjusted for a maximum E2, the quantity 22 may be, with sufficient accuracy, assumed as equal to unity in Equation 2, and the equation for maximum E2 reduces to:

The frequency to which L2C2 is tuned is designated as w1, and it is to be noted that for impressed frequencies higher than w1 leaving C2 fixed, the quantity (1-fp22) in the denominator of Equation (2) becomes negative, whereas, for frequencies lower than w1, it is positive.

Comparing Equation 2 with Equation 1, it is 'f apparent that for frequencies higher than w1 and with l- 22) negative, the phase of E3 will be approximately opposite to E2, provided M2 has the same sign as Mz.

for M2 and M3 of opposite sign;

Equations 5 show that the impressed frequency w1 at which maximum attenuation occurs bears a constant ratio to the frequency, w1, for maximum output E. That is, w1 corresponds to 22=1, approximately. The frequency w1 of maximum attenuation is given by Equations 5 so that the ratio of suppressed frequency w1 to desired frequency w1 is:

@1 M2 w1 N/l iMa An examination of Equation 6 will show that the frequency of maximum attenuation w1 bears a constant ratio to the desired transmission frequency w1 when the only couplings between the several coils are the constant magnetic couplings M2, M2. Coupling systems of this type may be satisfactory for some purposes but, in superheterodyne receivers employing a fixed intermediate frequency, the image frequency which is to be subjected to the maximum attenuation is spaced by a constant frequency interval from and is usually higher than the desired frequency w1. The coupling circuit must then be so designed that the frequency of maximum attenuation is equal to the desired frequency w1 plus a constant frequency interval equal to twice the intermediate frequency. To attain this result, it is evident that the ratio must become smaller as condenser C2 is tuned for maximum transmission at higher frequencies. Inspection of Equation 6 shows that the only possible variables are the couplings M2, M2 and that either M2 must decrease with increasing frequency or M3 must increase.

Reverting to Fig. l which illustrates the lrst alternative, it will be seen that the effect of the capacitive coupling C1 is to decrease the effect of the coupling M2 when that coupling is positive. Current flow through capacity C1 tends to reduce the voltage Ed from its value due to the positive magnetic coupling M2, and to reduce it to a greater extent with increasing frequency.

By a suitable choice of the relative values of the capacitive coupling and the two magnetic couplings, the system shown in Fig. 1 will yield a maximum attenuation at a frequency differing from that at which it aii'ords a maximum transmission by a substantially constant amount, and over an extended range of desired frequencies corresponding to the tuning of the circuit L2C2.

It is to be noted that the circuit shown in Fig. l will provide a maximum suppression for an image frequency lower than the desired frequency when couplings M2, M3 are of opposite sign, i. e., when coupling M3 is a negative mutual inductance. From an inspection of Equations 5, it will be apparent that the maximum suppression will be at an image frequency higher than the desired frequency when the magnetic couplings are of the same sign.

The circuit diagram, Fig. 1a, illustrates an arrangement for effecting the alternative procedure of obtaining a constant frequency difference between the desired and image frequency by effectively increasing the voltage E3 at higher frequencies. The capacitive coupling due to condenser C1 supplements the effect of the coupling M3 when the latter is negative and, if the image frequency is higher than the desired frequency, the coupling M2 must also be negative but, for suppression of an image frequency lower than the desired frequency, the coupling M2 should be positive.

As shown in Fig. 2, the coupling system of Fig. l may be employed between an antenna or other appropriate collector structure 1 and either a radio frequency amplifier or a rst detector 2 of a superheterodyne receiver. As noted above, the coupling system is reversible and the output terminals B, B1 are connected to the antenna by any convenient form of coupling such as the well-known series condenser method indicated by C0 and to ground 3, respectively, while the input terminals A, A1 are connected to the respective input terminals of the tube 2. The coil L1 may be tuned by a variable condenser C with this arrangement, the magnetic and capacitive couplings will usually be made smaller than in the case of an untuned L1 circuit in order that the system may be mainly responsive only to a relatively narrow band of frequencies.

As shown in Fig. 3, the novel coupling system may be employed as the interstage coupling between an amplifier tube 4 and the rst detector 5 of a superheterodyne receiver. The exact details of the circuit arrangement are subject to the usual variations of design but, as illustrated, the local oscillations from a source 6 are introduced between the cathode of the detector and the common or grounded terminal of the coupling system. A radio bypass condenser 7 is provided for grounding the coil L1 for carrier frequencies while permitting a feed of direct current to the plate of tube 4 from an appropriate source, indicated by the character EB. As noted in connection with Fig. 2, the plate coil L1 may, if desired, be tuned to the desired frequency by a variable condenser, not shown, which is preferably mechanically connected to the tuning condenser Ca of the I12C2 circuit.

The extreme simplicity of the physical equipment will be apparent from Fig. 4 which is a longitudinal section of an assembly suitable for use in the circuit of Fig. 3. Inductances L2, L3 are the usual single layer coils wound upon a common cylindrical form, the coil L2 being substantially identical with one which would be used, when tuned by a section C2 of a gang condenser, in a conventional radio coupling circuit. Coil L1 is a relatively high inductance of the universal wound type, and is coupled rather closely to L3. The capacity C1 is conveniently formed of a thin strip 9 of resilient metal adjacent the high potential or inside turns of the coil L1, and is adjustable by a screw 10 which is threaded into the header 11 that closes the form 8 and supports coil L1.

Coil La is of relatively few turns and the end adjacent coil L2, i. e., the intermediate terminal B2, is also connected to the strip 9 which forms a plate of condenser C1.

In one circuit arrangement of this type adapted for operation over the broadcast frequency from 550 to 1500 kc and designed to produce maximum attenuation at an image frequency 350 kilocycles higher than the desired frequency (intermediate frequency of kilocycles) the following values of circuit constants were used.

111:5 millihenries universal wound 1/8 thick on 1/2 dowel.

112:10() turns of #30 enamel wire, close wound on 11/4(l form, 233 microhenries.

L3=20 turns of #34 enamel wire close wound on the same form, 23.2 microhenries, and spaced Tag from end of L2.

M2=1S7-5 microhenries.

M3=130 microhenries.

Cizapproximately 1/2 micromicrofarads.

The value of coupling capacity C1 preferably is adjusted near the high frequency end of the tuning range to provide maximum attenuation to an image frequency of 1500 kilocycles, and the frequency at which maximum attenuation occurs will depart from its correct value only slight- 1y at lower frequencies. For example, when the circuit was tuned to a desired frequency of 800 kilocycles, the circuit gave maximum attenuation at 1135 kilocycles instead of at 1150 kilocycles. The tracking of the image suppression circuit is made most accurate preferably at the high frequencies since the normal circuit attenuation of the image is 'nere the least.

While the novel coupling or differential transmission system is particularly useful in superheterodyne receivers, it is to be understood that the invention is not limited thereto but is useful in a transmission system which the maximum attenuation is to be presented to a wave frequency which differs, in a predetermined relationship, from the desired frequency of maximum transmission.

I claim:

1. In an alternating current transmission network, an input circuit, an output circuit, one of said circuits including a resonant circuit in series with an inductance and the other circuit comprising an inductance, a magnetic coupling oetween the resonant circuit and the inductance of said second circuit, and substantially fixed impedance means constituting an additional coupling between said input and output circuits for effecting a maximum suppression of alternating currents of a frequency spaced from the resonant frequency of said resonant circuit.

2. In an alternating current transmission system, the invention as claimed in claim 1, wherein said resonant circuit includes a fixed inductance and an adjustable condenser for tuning said resonant circuit over a band of frequencies, thereby to determine the desired frequency of maximum transmission of said network.

3. In an electrical transmission system, the invention as set forth in claim 1, wherein said couplings are of such relative algebraic sign that the frequency of said spaced frequency is higher than the natural frequency of said resonant circuit.

4. In an electrical transmission, the invention as set forth in claim 1, wherein said couplings are of such relative algebraic sign that the frequency of said spaced frequency is lower than the natural frequency of said resonant circuit.

5. In an electrical transmission network, the invention as set forth in claim 1, wherein said resonant circuit includes a variable capacity for tuning said network for a maximum transmission at a desired frequency within a band of frequencies, and the frequency of said spaced frequency bears a substantially constant ratio to the desired frequency of transmission as the latter is varied over the said frequency band.

6. In an electrical transmission network, the invention as set forth in claim 1, wherein said resonant circuit includes a variable capacity for tuning said network for a maximum transmission at a desired frequency within a band of frequencies, in combination with an additional coupling between said input and output circuits for modifying the relative effect of said first two couplings to maintain a substantially constant frequency interval between said spaced frequency and said desired frequency as the latter is varied over the said frequency band by adjustment of said capacity.

'7. In an electrical network for the selective transmission of alternating current signals, a pair of circuits serially arranged in the direction of signal transmission, one of said circuits including a pair of serially arranged inductances and the other circuit including an inductance, capacity shunting at least one of said inductances for determining the desired frequency at which maximum transmission is to be effected, and a plurality of relatively fixed couplings between the several inductances for effecting a maximum suppression of signals at frequency spaced from the said desired frequency.

8. In an electrical network, the invention as claimed in claim 7, wherein said capacity is shunted across one of said serially arranged inductances.

9. In an electrical network, the invention as claimed in claim 7, wherein said capacity is shunted across one of said serially arranged inductances, and said couplings include-a magnetic coupling between said capacity-shunted inductance and the inductance of the other circuit.

10. In an electrical transmission network, the invention as claimed in claim 7, wherein said capacity is adjustable for varying the desired frequency of maximum transmission over a band of frequencies, and said plurality of relatively fixed couplings include couplings of opposite types and of relative magnitudes to effect a maximum suppression at a frequency which differs from the said desired frequency by a substantially constant value as the latter is varied over said frequency band by adjustment of said capacity.

11. In an electrical transmission network, the combination with a pair of input terminals and a pair of output terminals, of a pair of serially connected inductances between one pair of terminals, a third inductance between the other pair of terminals, a condenser shunted across one of said serially connected inductances for determining the desired frequency of maximum transmission, mutual inductance between said condensershunted inductance and said third inductance for the transfer of energy at the said desired frequency, and mutual inductance between the second of said pair of inductances and said third inductance for neutralizing the transfer of energy by said first mutual inductance at a frequency spaced from said desired frequency.

12. In an alternating current transmission network, a coupling system having two input and two output terminals, one input and one output terminal being at substantially the same alternating current potential, the combination with an inductance between one pair of terminals and a pair of serially connected inductances between the other terminals, a tuning condenser shunted across one of said serially connected inductances, mutual inductance between said first inductance and each of said serially connected inductances, and a relatively fixed capacitive coupling between the junction of said serially connected inductance and that terminal of the first inductance which is not at substantially the same alternating current potential as one terminal of said serially connected inductances.

13. In a frequency selective network for the transmission of alternating currents, a pair of terminals, an untuned inductance between said terminals, a second pair of terminals and an electrical connection between said pair of terminals to maintain a terminal of one pair at substantially the same alternating potential as a terminal of the other pair, two inductances serially connected between said second pair of terminals, mutual inductance between said first inductance and each of said pair of inductances, and a tuning condenser shunting one of said pair of inductances.

14. In a frequency selective network, the invention as set forth in claim 13, wherein said mutual inductances are of the same algebraic sign.

15. In a frequency selective network, the invention as set forth in claim 13, wherein said mutual inductances are of opposite algebraic sign.

16. In an electrical transmission system for alternating currents, the combination with a pair of terminals adapted to be connected respectively to a collector structure and to ground, and a second pair of terminals adapted to be connected to the input elements of a vacuum tube, an inductance between one pair of terminals, a pair of inductances serially connected between the other pair of terminals, a condenser shunting one of said pair of inductances for determining the desired frequency of maximum transmission of said network, mutual inductance between said first inductance and one of said serially connected inductances and a second mutual inductance between said flrst inductance and the second of said serially connected inductances for suppressing the transmission of alternating currents of a frequency spaced from said desired frequency.

17. In an electrical transmission system, the invention as set forth in claim 16, in combination with a capacitive coupling between said first inductance and said serially connected inductances for varying with adjustments of said desired frel quency the ratio of the said spaced frequency of suppression to the said desired frequency.

18. In an electrical transmission system of the type including cascaded vacuum tubes, a plate circuit for one tube and an input circuit for the succeeding tube, one circuit including an inductance and the other including a pair of serially connected inductances, an adjustable capacity shunting one of said serially connected inducv tances for determining the desired frequency of maximum transmission, mutual inductance between said first inductance and said capacityshunted inductance, and mutual inductance between said rst inductance and said other serially connected inductance for neutralizing the transmission of energy of a frequency spaced from that of said desired frequency.

19. In an electrical transmission system, the invention as set forth in claim 18, in combination with a relatively fixed capacitive coupling between said circuits for effecting a maximum suppression at a frequency spaced from said desired frequency by a substantially constant value as said desired frequency is varied over a frequency band by adjustment of said capacity.

20. In the selective transmission of alternating currents by a network including two circuits including inductance, and in which at least one of the circuits includes an inductance tuned to the desired frequency of maximum transmission, the method of suppressing the transmission of alternating currents of an undesired frequency spaced from said desired frequency which comprises coupling said circuits to effect transmission at the desired frequency with a lesser and undesired transmission at the undesired frequencies, and providing an additional untuned coupling between said circuits to neutralize the transmission by the first coupling at the undesired frequency.

21. In the selective transmission oi' alternating currents by a network including two serially arranged circuits, one circuit including an inductance and the other circuit including an untuned inductance in series with an inductance tuned to the desired frequency of maximum transmission by a tuning condenser, the method of suppresssing transmission of alternating currents of an undesired frequency which comprises coupling the inductance of the first circuit to the tuned inductance of the second circuit to effect transmission at the desired frequency with a lesser and undesired transmission at the undesired frequency, and coupling the first circuit to the untuned inductance of the second circuit to neutralize the transmission oy the first coupling at the undesired frequency.

22. The invention as claimed in claim 21, wherein the second coupling is varied with the tuning of said tuned inductance to maintain a constant frequency interval between the desired frequency and the frequency which is suppressed by the second coupling.

23. In the selective transmission of alternating currents by a network including two serially arranged circuits, one circuit including an untuned inductance and the other circuit including an untuned inductance in series with an inductance tuned to the desired frequency of maximum transmission by a tuning condenser, the method of suppressing transmission of alternating currents of an undesired frequency which comprises coupling the inductance of the first circuit to the tuned inductance of the second circuit to effect transmission at the desired frequency with a lesser and undesired transmission at the undesired frequency, and coupling the iirst circuit to the untuned inductance of the second circuit to neutralize the transmission by the first coupling at the undesired frequency.

PAUL O. FARNHAM. 

